1. Field of the Invention
The present invention relates to a semiconductor device that includes a wiring board and an integrated circuit (IC) chip mounted thereon and, particularly, to a heat dissipation structure in a semiconductor device.
2. Description of Related Art
The heat value of an IC chip such as a liquid crystal driver IC during operation is on the increase. The increase in the IC chip heat value is particularly significant in a semiconductor device using a flexible wiring board compared to a semiconductor device using a rigid wiring board. Therefore, a heat dissipation structure in a semiconductor device becomes increasingly important.
A flexible wiring board has a structure in which a conductor layer such as lines is formed on a flexible insulating film. For example, the conductor layer is formed by applying copper foil to a polyimide film, which is the insulating film. The flexible wiring board has a lower heat capacity and a lower mechanical strength compared to the rigid wiring board for its thin profile. It is thus generally difficult to mount heavy parts such as a heat sink on the flexible wiring board.
In order to deal with the heat issue when using a flexible wiring board, Japanese Unexamined Patent Application Publications Nos. 2007-158001 and 2004-111996 disclose a heat dissipation structure of a tape carrier package (TCP), which is one type of a semiconductor device using a flexible wiring board. The TCP disclosed in Japanese Unexamined Patent Application Publication No. 2007-158001 (JP 2007-158001 A) has the following structure. Namely, an IC chip that has an electrode for heat dissipation separately from an electrode used for signal input/output with an external device is mounted on a flexible wiring board. Further, a heat dissipation conductive pattern that is physically isolated from a signal line pattern is formed on the surface of the flexible wiring board. The heat dissipation electrode of the IC chip and the heat dissipation conductive pattern of the wiring board are connected through a conductor (gold bump, solder etc.). In this specification, connection through a conductor such as gold bump or solder is referred to as “conductor connection”.
Japanese Unexamined Patent Application Publication No. 2004-111996 (JP 2004-111996 A) also discloses a technique that forms a heat dissipation conductive pattern on the surface of a flexible wiring board for the purpose of heat dissipation of an IC chip mounted on the flexible wiring board. However, in JP 2004-111996 A, no electrodes of the IC chip and the heat dissipation conductive pattern are connected. Specifically, JP 2004-111996 A discloses a structure in which the IC chip and the heat dissipation conductive pattern are physically spaced (FIG. 3 in JP 2004-111996 A) and a structure in which the IC chip and the heat dissipation conductive pattern are physically contacted (FIG. 6 in JP 2004-111996 A). More specifically, FIGS. 3 and 6 of JP 2004-111996 A show structures in which no electrodes of the IC chip and the heat dissipation conductive pattern are in conductor connection and the heat dissipation conductive pattern is formed opposite to all over the length of the short side of the rectangular IC chip.